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Register a new userRaman spectroscopy of rare-earth orthoferrites RFeO3 (R = La, Sm, Eu, Gd, Tb, Dy)
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We report a Raman scattering study of six rare earth orthoferrites RFeO3, with R = La, Sm, Eu, Gd, Tb, Dy. The use of extensive polarized Raman scattering of SmFeO3 and first-principles calculations enable the assignment of the observed phonon modes to vibrational symmetries and atomic displacements. The assignment of the spectra and their comparison throughout the whole series allows correlating the phonon modes with the orthorhombic structural distortions of RFeO3 perovskites. In particular, the positions of two specific Ag modes scale linearly with the two FeO6 octahedra tilt angles, allowing the distortion throughout the series. At variance with literature, we find that the two octahedra tilt angles scale differently with the vibration frequencies of their respective Ag modes. This behavior as well as the general relations between the tilt angles, the frequencies of the associated modes and the ionic radii are rationalized in a simple Landau model. The reported Raman spectra and associated phonon-mode assignment provide reference data for structural investigations of the whole series of orthoferrites.
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We report on optical studies of the thin films of multiferroic hexagonal (P.G. 6mm) rare-earth orthoferrites RFeO3 (R=Ho, Er, Lu) grown epitaxially on a (111)-surface of ZrO2(Y2O3) substrate. The optical absorption study in the range of 0.6-5.6 eV shows that the films are transparent below 1.9 eV; above this energy four broad intense absorption bands are distinguished. The absorption spectra are analyzed taking into account the unusual fivefold coordination of the Fe(3+) ion. Temperature dependence of the optical absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic ordering of iron sublattices.
Using Fourier-transform infrared spectroscopy and optical ellipsometry, room temperature spectra of complex conductivity of single crystals of hexaborides Gd$_x$La$_{1-x}$B$_6$, $x$(Gd)$=0$, 0.01, 0.1, 0.78, 1 are determined in the frequency range 30$-$35000$~cm^{-1}$. In all compounds, in addition to the Drude free-carrier spectral component, a broad excitation is discovered with the unusually large dielectric contribution $Delta$$varepsilon$=5000 -- 15000 and non-Lorentzian lineshape. It is suggested that the origin of the excitation is connected with the dynamic cooperative Jahn-Teller effect of B$_6$ clusters. Analysis of the spectra together with the results of DC and Hall resistivity measurements shows that only 30$-$50$%$ of the conduction band electrons are contributing to the free carrier conductivity with the rest being involved in the formation of an overdamped excitation, thus providing possible explanation of remarkably low work function of thermoemission of Gd$_x$La$_{1-x}$B$_6$ and non-Fermi-liquid behavior in GdB$_6$ crystals.
The effect of rare-earth ion size on the octahedral distortions in rare-earth chromites (RCrO3, R = Lu, Tb, Gd, Eu, Sm) crystallizing in the orthorhombic structure has been studied using Raman scattering and synchrotron powder x-ray diffraction up to 20 GPa. From our studies on RCrO3 we found that the octahedral tilts (distortions) increase with pressure. This is contrary to the earlier report which suggests that in LaCrO3, the distortions decrease with pressure leading to a more ordered phase at high pressure. Here we observe that the rate of increase in distortion decreases with the increase in R-ion radii. This occurs due to the reduction in the compression of RO12 polyhedra with a corresponding increase in the compression of the CrO6 octahedra with increasing R-ion radii. From the Raman studies, we predict a critical R-ion radii, above which we expect the distortions in RCrO3 to reduce with increasing pressure leading to what is observed in the case of LaCrO3. These Raman results are consistent with our pressure dependent structural studies on RCrO3 (R = Gd, Eu, Sm). Also, our results suggest that the pressure dependence of Neel temperature, TNCr, (where the Cr3+ spin orders) in RCrO3 is mostly affected by the compressions of Cr-O bonds rather than the alteration of octahedral tilts.
Magnetic circular dichroism (MCD) in the x-ray absorption spectroscopy (XAS) at the L2,3 edges for almost entire series of rare-earth (RE) elements in RE2Fe14B, is studied experimentally and theoretically. By a quantitative comparison of the complicated MCD spectral shapes, we find that (i) the 4f-5d intra-atomic exchange interaction not only induces the spin and orbital polarization of the 5d states, which is vital for the MCD spectra of the electric dipole transition from the 2p core states to the empty 5d conduction band, but also it accompanies a contraction of the radial part of the 5d wave function depending on its spin and orbital state, which results in the enhancement of the 2p-5d dipole matrix element, (ii) there are cases where the spin polarization of the 5d states due to the hybridization with the spin polarized 3d states of surrounding irons plays important roles, and (iii) the electric quadrupole transition from the 2p core states to the magnetic vale! nce 4f states is appreciable at the pre-edge region of the dipole spectrum. Especially, our results evidence that it is important to include the enhancement effect of the dipole matrix element in the correct interpretation of the MCD spectra at the RE L2,3 edges.
The antiferromagnetic transition is investigated in the rare-earth (R) tritelluride RTe3 family of charge density wave (CDW) compounds via specific heat, magnetization and resistivity measurements. Observation of the opening of a superzone gap in the resistivity of DyTe3 indicates that additional nesting of the reconstructed Fermi surface in the CDW state plays an important role in determining the magnetic structure.
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